天高くハム肥ゆる10月

This is the title of a manga chapter I'm trying to translate, one of a series of omakes each set in a different month hence the 10月 at the end.

The omake storyline involves high-school baseball players comparing their muscles, particularly their hamstrings so I think that's where the "ハム" comes in but I'm lost for the rest of it. Can anyone help?

It's a pun for the idiom "天高く馬肥ゆる秋" (Autumn that the sky gets higher and horses grow fat), that describes an idyllic landscape in autumn. Not a colloquial expression but still commonly-used in newspapers or seasonal letters.

By the way, this idiom is originally a quote from the poem by the Chinese poet 杜審言. In the original text "horses" were the enemies' warhorses and "to grow fat" meant they get ready to fight, so the situation was not really "idyllic"

Thank you. Sadly my British education and upbringing did not include much in the way of classical Japanese and Chinese literature so it wasn't obvious to me. In medieval Europe the usual time to make war was autumn when the harvest had been gathered in but the weather was still good enough for marches and movement with dry ground and mild weather. I presume this was also true in Japan.

How does "Hams grow fat under autumn skies: October" seem to you as a possible title? I would explain the pun later in my translation.

This is about the autumn's peacefulness and its nice weather, as Arui said. Autumn is a harvest season, yes as you said; you can get nice foods a lot. See this says that horses grow fat. Therefore, in the modern popular usage, this implies that you gain the weight in autumn. No joke.
So I guess that this parody suggests their hamstrings grow bigger and bigger during the October, according to your post. Or I thought that someone fat called ham (food) is getting fatter then. But this is less likely.